A few questions.....
Does the base metal appear to be silver or copper? The side next to the dime appears to have a silver hue, but I generally equate a green encrustation to be copper.
What general Florida beach was this found at?
Also, take a magnet and see if it is ferrous....on many of the treasure beaches in Florida, iron flaking from ships washes ashore, although I don't think that is the case.
The marking is certainly there, so I think it is a coin....just not sure if it is somewhat modern or a reale. Where you found it and the metal content are the key for further investigation.
Whatever it is, it appears to be thin and fragile..In the past I have dipped something like that in Muriatic Acid for 30 seconds after wrapping it tightly in aluminum foil, then wash in fresh water after unwrapping it, and detail might come up...definitely not the preferred restoration method, but in this case it will be more of a keepsake than to hold numismatic value.
With a second look at it, your second pic seems to show the outline of the Lincoln Memorial...likely a Lincoln cent. Copper plated zinc cents deteriorate very quickly in salt water.
goldcoastwayne
This method will give you some answers on composition....
